Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when the window pane is being directed to the closed position.
One proximity sensor commonly used as a power window controller comprises a voltage sensor coupled between the window actuator and the actuator power source. When an obstruction is encountered in the window frame as the window is closing, the obstruction increases the electrical load imposed on the window actuator, thereby causing the load voltage at the window actuator to drop. The voltage sensor is configured to sense any drop in load voltage and to command the window actuator to stop or to reverse the direction of movement of the window pane when such a voltage drop is detected.
Another proximity sensor commonly used as a power window controller comprises a speed sensor coupled to the window actuator. When an obstruction is encountered in the window frame as the window is closing, the obstruction increases the mechanical load imposed on the window actuator, thereby causing the speed of the actuator to drop. The speed sensor is configured to sense any change in actuator speed and to command the window actuator to stop or to reverse the direction of movement of the window pane when such a change in actuator speed is detected.
Another proximity sensor employed comprises a pressure sensitive strip disposed around the upper edge of the window frame. When an obstruction is detected between the window pane and the window frame, the pressure sensitive strip signals the window actuator to stop further movement of the window pane.
Although voltage sensors, speed sensors and pressure sensors are commonly used in power window controllers, they cannot react with sufficient speed to prevent an obstruction, such as a hand, from being pinched between the window pane and the window frame. Consequently, attempts have been made to improve upon the conventional proximity sensor mechanism.
For instance, Peter (U.S. Pat. No. 5,801,340) teaches a solution which uses a capacitive sensor mounted on the weather seal at the top of the window frame. The capacitive sensor comprises a first insulating layer disposed over the window sheet metal, a conductive guard layer disposed over the first insulating layer, a second insulating layer disposed over the guard layer, and a touch plate disposed over the second insulating layer. The guard layer is driven by an alternating voltage signal which is identical in amplitude and phase to the voltage imposed on the touch plate. With this arrangement, capacitance between the touch plate and the window sheet metal is cancelled out, thereby increasing the sensitivity of the sensor to capacitive changes arising from obstructions in the window frame.
Although Peter allows the window actuator to respond more rapidly to obstructions in the window frame, Peter requires that the guard layer be the same size as the touch plate for optimum cancellation of touch plate capacitance. In fact, Peter points out that if the guard plate extends beyond the touch plate, the sensitivity of the sensor to obstructions will be reduced. Consequently, Peter discloses that the guard layer extends only  10/1000th of an inch beyond the touch plate. The disclosed manufacturing tolerances can greatly increase the manufacturing cost of the capacitive sensor. Consequently, there remains a need for a proximity sensor which will not allow an obstruction to become pinched between the window pane and the window frame when the window pane is being directed to the closed position. 